Updated documentation for MetaData

doc/plugin-metadata.markdown

@@ -7,7 +7,7 @@ Metadata are custom fields, it's a key/value table.
 For example your plugin can store external information for a task or new settings for a project.
 Basically that allow you to extend the default fields without having to create new tables.
 
-Attach metadata to tasks
+Attach metadata to tasks and remove them
 ------------------------
 
 ```php
@@ -23,6 +23,9 @@ $this->taskMetadata->exists($task_id, 'my_plugin_variable');
 
 // Create or update metadata for the task
 $this->taskMetadata->save($task_id, ['my_plugin_variable' => 'something']);
+
+// Remove a metadata from a project
+$this->projectMetadata->remove($project_id, my_plugin_variable);
 ```
 
 Metadata types